Thereafter, 2 netbooks (XP SP3) are unable to connect to the Internet as the network no longer shows up in the list of wireless networks. A refresh does not help.

I can connect a netbook to the Modem/Router via an Ethernet cable and all is OK.

On a notebook, if I right click the Wireless Network Connection and go to the Wireless Networks tab, my network name does show up in the Preferred Networks list (2nd entry - the first entry is the main Study PC (Vista - latest SP) which is not wireless).

Well, two things come to mind straight away. Could the reboot have lost your wireless settings and now the wireless setup is not enabled? I would check in the web interface and triple-check that all the settings for wireless are configured. If wireless is enabled - ensure that your SSID is configured how you wanted it and that "Hide SSID" is not ticked (if the option exists).

Thank you for your suggestions and I have it back up and running. Fortunately, I had screen-shots of all my router screens. The only thing I changed back was to do with the DNS Server. About 2-3 weeks ago I changed Auto Discovery from Auto to User (and entered the OpenDNS addresses) after reading an article. By changing it back to Auto and rebooting all the computers and router (whether they needed it or not) has solved the problem.